Prompt emission of high-energy photons from gamma ray bursts

Abstract

Within the internal shock scenario, we consider different mechanisms of high-energy (>1 MeV) photon production inside a gamma ray burst (GRB) fireball and derive the expected high-energy photon spectra from individual GRBs during the prompt phase. The photon spectra of leptonic and hadronic origins are compared within different sets of parameter regimes. Our results suggest that the high-energy emission is dominated by the leptonic component if the fraction of shock energy carried by electrons is not very small (e.g. ɛe > 10-3). For very small values of ɛe, the hadronic emission component could be comparable to or even exceed the leptonic component in the GeV-TeV regime. However, in this case a much larger energy budget of the fireball is required to account for the same level of the observed sub-MeV spectrum. The fireballs are therefore extremely inefficient in radiation. For a canonical fireball bulk Lorentz factor (e.g. Γ = 400), emissions above ~10 GeV are attenuated by two-photon pair production processes. For a fireball with an even higher Lorentz factor, the cut-off energy is higher, and emissions of 10 TeV-PeV due to π0-decay can also escape from the internal shocks. The flux level is, however, too low to be detected by current TeV detectors, and these photons also suffer attenuation by external soft photons. The GLAST Large Area Telescope can detect prompt emission of bright long GRBs above 100 MeV. For short GRBs, the prompt emission can be only barely detected for nearby bright ones with relatively `long' durations (e.g. ~1 s). With the observed high-energy spectrum alone, it appears that there is no clean picture to test the leptonic versus hadronic origin of the gamma rays. Such an issue may be, however, addressed by collecting both prompt and afterglow data. A moderate-to-high radiative efficiency would suggest a leptonic origin of high-energy photons, while a GRB with an extremely low radiative efficiency but an extended high-energy emission component would be consistent with (but not a proof for) the hadronic origin.